The present invention relates to a vertical type heat treatment system for heat treatment a plurality of semiconductor wafers at once and, more particularly, to a system as a combination of a gas shower mechanism for blowing a gas to wafers loaded/unloaded into/from a furnace, a gas circulating/cooling/cleaning system in a boat section, and a pass box for loading/unloading wafers into/from a cassette.
For example, in a semiconductor device manufacturing process, various types of processing apparatuses are used for forming an oxide film on a semiconductor wafer, a thin film in accordance with thermal CVD, a heavily doped region in accordance with thermal diffusion, and the like.
Heat treatment apparatuses adopted by these various types of processing apparatuses have recently been shifted from conventional horizontal ones to vertical ones. The vertical heat treatment apparatuses will be described hereinafter.
In such a vertical type heat treatment system, when a carrier (cassette) housing a large number of semiconductor wafers aligned in order is introduced to the loading/unloading area of the apparatus body, this carrier is placed on a transfer stage by, e.g., a carrier transfer. The semiconductor wafers in the carrier on the transfer stage are transferred to the wafer boat at the loading area one by one or in units of five wafers and stored and held in a multiple of stages. In this state, the wafer boat is moved upward by a boat elevator to load the semiconductor wafers into a process tube. The process tube is sealed and heated while substituting its internal gas with a predetermined process gas, thereby performing a required processing operation to the semiconductor wafers.
The processed semiconductor wafers are moved downward by the boat elevator from the interior of the process tube together with the wafer boat and withdrawn to the loading area immediately under the process tube. The semiconductor wafers are then returned from the loading area into the carrier on the transfer stage by the wafer transfer, and removed outside the apparatus body or conveyed to a connected processing apparatus of a subsequent process from the loading/unloading area together with the carrier.
In this processing operation by the vertical type heat treatment system, when the semiconductor wafers are moved upward together with the wafer boat from the loading area (boat section) and loaded into the process tube, or are moved downward from the process tube after processing in the process tube and withdrawn, the vicinity of a furnace port located midway along this process is set in a considerably high temperature atmosphere. If the outer air exists at this portion, a spontaneous oxide film is undesirably formed on the surface of the semiconductor wafer by the O.sub.2 gas in the outer air. For this reason, to perform the boat loading/unloading operation in an inert gas atmosphere (non-oxygen atmosphere) of, e.g., N.sub.2 gas, the interior of the apparatus body, especially the loading area, is desired to be substituted with and maintained in a N.sub.2 gas atmosphere as a closed system structure separated from the outer air by a gas supply/exhaust means.
Furthermore, the gas atmosphere in the apparatus body must always be held at a positive pressure so as to prevent the outer air from entering in it. Since the semiconductor wafer processing operation is repeatedly performed, gaseous impurities, e.g., carbon, and particle impurities, e.g., oil mist or dust, are formed in the gas atmosphere in the apparatus body. These impurities are deposited on the semiconductor wafers or cause chemical contamination to degrade the characteristics and yield of the semiconductor devices. Therefore, it is desired that a clean inert gas is always introduced into the apparatus body as a purge gas, thereby maintaining the inert gas atmosphere in the apparatus body at a high-purity positive pressure.
However, even when the vertical type heat treatment system has the closed system structure as described above, it is difficult to maintain the interior of the apparatus body, especially the loading area in a high-purity inert gas atmosphere. When the semiconductor wafers are loaded/unloaded into/from the process tube while they are held in the wafer boat in a multiple of stages, gaseous impurities, e.g., O.sub.2, and particle impurities exist among the semiconductor wafers, or heat gets involved, so that the impurities are deposited on the semiconductor wafers or cause chemical contamination (formation of a natural oxide film, and the like) leading to a defect, thereby degrading the characteristics and yield of the semiconductor elements.